Generally, a drill tool comprises a cylindrical drill body and two indexable inserts detachably mounted to an end portion of the drill body. The two indexable inserts consist of an inner insert mounted adjacent to a central axis of the drill body and an outer insert mounted adjacent to a periphery of the drill body. Further, they are commonly square-shaped inserts with four cutting edges as shown in FIG. 1. The inner insert and the outer insert are arranged so that the traces of two rotating inserts are overlapped. More specifically, an internal diameter of the circular area of a work piece processed by the inner insert is a bit larger than an external diameter of the ring-shaped area processed by the outer insert.
It is preferable that the ratio of effective cutting edge length of the inner insert to effective cutting edge length of the outer insert satisfies 50:50. Otherwise, vibrations during a processing operation are intensified and may negatively affect the processing performance of the drill due to the imbalanced force transmitted to the drill tool through the inner and outer inserts.
In an indexable insert drill for processing small diameter holes below 14 mm, there is a technical difficulty in realizing the above-mentioned ratio of the effective cutting edge lengths of the inner insert to the outer insert of 50:50 with the conventional square-shaped inserts. The scale of a cemented carbide tool cannot be unlimitedly miniaturized due to its properties. For example, reducing the amount of a material for the size of the insert to be smaller may weaken the insert. Further, the insert should have a space for clamping a screw to fix the indexable insert to the drill body. As such, in designing a drill tool of small diameter with square-shaped inserts with more than the desired size to realize the ratio of the effective cutting edge lengths of the inner insert to the outer insert to be 50:50 as shown in FIG. 1, a diameter of the inscribed circle is 4.5 mm.
A technique used to drill small diameter holes by using rectangular-shaped inserts (rather than square-shaped inserts) to realize the ratio of the effective cutting edge lengths of 50:50 has been provided. It can ensure a sufficient width around a screw hole of the insert and in an end portion of the drill tool to maintain a sufficient stiffness. However, it has drawbacks since the rectangular-shaped insert can only use two cutting edges, whereas the square-shaped insert can use four cutting edges. Further, the strength of the rectangular-shaped insert is inferior to that of the square-shaped insert.